1. Field of the Invention
The present invention relates to sensors for detecting an analyte in liquid using SAW elements (surface acoustic wave elements), and also relates to devices for detecting an analyte in liquid including such sensors. More specifically, the present invention relates to sensors for detecting an analyte in liquid, each sensor includes at least one SAW element mounted on a base substrate via a bump electrode, and also relates to devices for detecting an analyte in liquid with the sensors.
2. Description of the Related Art
Various types of sensors for detecting analytes in liquids have been disclosed in the past.
For example, Japanese Unexamined Patent Application Publication No. 63-250560 (Patent Document 1) discloses a sensor for detecting an analyte in liquid using a surface acoustic wave. FIG. 23 is a schematic front cross-sectional view for illustrating the sensor disclosed in Patent Document 1.
A sensor 102 for detecting an analyte in liquid is immersed in a solution 101 containing an analyte. The sensor 102 is defined by a surface acoustic wave element. In other words, the sensor 102 includes a rectangular plate-like piezoelectric substrate 103, an input IDT electrode 104, and an output IDT electrode 105. The input IDT electrode 104 and the output IDT electrode 105 are arranged on the same surface of the piezoelectric substrate 103 with a predetermined distance therebetween. In addition, a membrane 106 for adsorbing an analyte is arranged between the input IDT electrode 104 and the output IDT electrode 105. A surface acoustic wave is excited in the piezoelectric substrate 103 by applying an alternating voltage to the input IDT electrode 104. The excited surface acoustic wave propagates toward the output IDT electrode 105. At the output IDT electrode 105, an electric signal based on the propagated surface wave is extracted. Since the membrane 106 adsorbs the analyte, the load to the surface of the piezoelectric substrate 103 due to the membrane 106 is changed when the analyte is present. Consequently, the propagating surface acoustic wave is modified by the presence of the analyte and, therefore, the output extracted from the output IDT electrode 105 is changed. Thus, the detection of the analyte and the measurement of concentration thereof are enabled.
However, in the measurement method using the sensor 102 for detecting an analyte in liquid, the sensor 102 must be immersed in a liquid 101. Therefore, when the amount of the liquid 101 containing the analyte is small, the analyte in the liquid cannot be detected.
Further, even if a large amount of a liquid is prepared, the measurement cost is disadvantageously high when the liquid is expensive.
In addition, in the sensor 102 for detecting an analyte in liquid, a liquid 101 adheres not only to regions where the surface acoustic wave propagates but also to regions where electrode pads or bonding wires, which are connected to the IDT electrodes 103 and 104, are disposed. Therefore, the electrical characteristic is disadvantageously changed so as to deteriorate the detection accuracy.
On the other hand, Japanese Unexamined Patent Application Publication No. 5-45339 (Patent Document 2) discloses a method for measuring an analyte in liquid without immersing a sensor for detecting an analyte in liquid.
In the sensor of Patent Document 2, an IDT electrode is disposed on a first principle surface of a piezoelectric substrate, and a measuring pond for receiving a liquid containing an analyte is formed on a second principle surface of the piezoelectric substrate, which is the opposite side of the first principle surface. Here, a liquid is injected into the measuring pond provided on the second principle surface, and the measurement is performed. Thus, it is not necessary to immerse the entire sensor in the liquid. Furthermore, since the IDT electrode is not brought into contact with the liquid, the electrical characteristic is not substantially changed.
As described above, in the sensor disclosed in Patent Document 2 for detecting an analyte in liquid, a large amount of liquid is not necessary. In addition, the liquid negligibly adheres to the IDT electrode.
However, in the sensor disclosed in Patent Document 2, a liquid containing an analyte is present on the second principle surface of the piezoelectric substrate. On the other hand, a surface acoustic wave propagates on the first principle surface of the piezoelectric substrate, namely, the surface acoustic wave propagates extremely close to the surface of the principle surface on which the IDT electrode is disposed. Therefore, when a solution is applied to the second principle surface, the surface acoustic wave propagating on the first principle surface is not substantially influenced by the change caused by the presence of the solution. Therefore, in the sensor disclosed in Patent Document 2, the detection accuracy cannot be sufficiently increased.
In addition, in the sensor disclosed in Patent Document 2, energy disperses not only to near the surface of the piezoelectric substrate but also to a certain depth of the piezoelectric substrate. The leak component of a propagating SH wave causes noise, and therefore, the measurement accuracy is disadvantageously decreased.